V Ramachandran and E.M. Hubbard .S.

Synaesthesia — A Window Into Perception, Thought and Language
Abstract: We investigated grapheme–colour synaesthesia and found that: (1) The induced colours led to perceptual grouping and pop-out, (2) a grapheme rendered invisible through ‘crowding’ or lateral masking induced synaesthetic colours — a form of blindsight — and (3) peripherally presented graphemes did not induce colours even when they were clearly visible. Taken collectively, these and other experiments prove conclusively that synaesthesia is a genuine perceptual phenomenon, not an effect based on memory associations from childhood or on vague metaphorical speech. We identify different subtypes of number–colour synaesthesia and propose that they are caused by hyperconnectivity between colour and number areas at different stages in processing; lower synaesthetes may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas higher synaesthetes may have cross-activation in the angular gyrus. This hyperconnectivity might be caused by a genetic mutation that causes defective pruning of connections between brain maps. The mutation may further be expressed selectively (due to transcription factors) in the fusiform or angular gyri, and this may explain the existence of different forms of synaesthesia. If expressed very diffusely, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor and synaesthesia (and the higher incidence of synaesthesia among artists and poets). Also, hyperconnectivity between the sensory cortex and amygdala would explain the heightened aversion synaesthetes experience when seeing numbers printed in the ‘wrong’ colour. Lastly, kindling (induced hyperconnectivity in the temporal lobes of temporal lobe epilepsy [TLE] patients) may explain the purported higher incidence of synaesthesia in these patients. We conclude with a synaesthesia-based theory of the evolution of language. Thus, our experiments on synaesthesia and our theoretical framework attempt to link several seemingly unrelated facts about the human mind. Far from being a mere curiosity, synaesthesia may provide a window into perception, thought and language.
Correspondence: Center for Brain and Cognition, University of California, San Diego, 9500 Gilman Dr. 0109, La Jolla, CA 92093-0109, e-mail: vramacha@ucsd.edu

Journal of Consciousness Studies, 8, No. 12, 2001, pp. 3–34

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V.S. RAMACHANDRAN & E.M. HUBBARD

Introduction Synaesthesia is a curious condition in which an otherwise normal person experiences sensations in one modality when a second modality is stimulated. For example, a synaesthete may experience a specific colour whenever she encounters a particular tone (e.g., C-sharp may be blue) or may see any given number as always tinged a certain colour (e.g., ‘5’ may be green and ‘6’ may be red). The condition was first clearly documented by Galton (1880) who also noted that it tends to run in families. One problem that has plagued research in this field is that, until recently, it was not even clear that synaesthesia is a genuine sensory/ perceptual phenomenon (Baron-Cohen & Harrison, 1997; Cytowic, 1989; Harrison, 2001; Ramachandran & Hubbard, 2001a). Indeed, despite a century of research, the phenomenon is still sometimes dismissed as bogus. We have frequently encountered the following types of explanations in the literature as well as in conversations with professional colleagues: 1) They are just crazy. The phenomenon is simply the result of a hyperactive imagination. Or maybe they are trying to draw attention to themselves by claiming to be special or different in some way. 2) They are just remembering childhood memories such as seeing coloured numbers in books or playing with coloured refrigerator magnets. 3) They are just engaging in vague tangential speech or just being metaphorical just as you and I might say ‘bitter cold’ or ‘sharp cheese’. Cheese is soft to touch, not sharp, so why do we say ‘sharp’? Obviously, one means that the taste is sharp but why is a tactile adjective being applied to taste? 4) They are ‘potheads’ or ‘acid junkies’ who have been on drugs. This idea is not entirely without substance since LSD users often do report synaesthesia both during the high as well as long after. Although common, none of these accounts provides a satisfactory explanation of synaesthesia. For example, the idea that synaesthetes are trying to draw attention to themselves would predict that synaesthetes should be telling everyone around them about how different they are. In our experience, it is usually quite the opposite. Synaesthetes often think that everyone else experiences the world the same way they do, or else they have been ridiculed as children and have not told anyone about their synaesthesia for years. The memory hypothesis also fails as an explanation of synaesthesia because it cannot address the questions of why only some individuals have these memories intact, why only specific classes of stimuli are able to induce synaesthesia, and why there should be a genetic basis for synaesthesia (see below). The problem with the metaphor explanation is that it commits one of the classical blunders in science, trying to explain one mystery (synaesthesia) in terms of another mystery (metaphor). Since we know very little about the neural basis of metaphor, saying that ‘synaesthesia is just metaphor’ helps to explain neither synaesthesia nor metaphor. Indeed, in this paper we will turn the problem on its head and suggest the very opposite: Synaesthesia is a concrete sensory phenomenon whose neural basis we are beginning to understand and it can therefore

SYNAESTHESIA — PERCEPTION, THOUGHT AND LANGUAGE

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provide an experimental lever for understanding more elusive phenomena such as metaphor (Ramachandran & Hubbard, 2001a). Finally, the idea that synaesthesia is a result of drug use is only applicable to a few people, and seems to occur only during the ‘trip’. One explanation of this is that certain drugs might pharmacologically mimic the same physiological mechanisms that underlie genetically based synaesthesia. However, it may also be that pharmacologically induced synaesthesia is not based on the same neural mechanisms as the congenital, lifelong experiences of true synaesthetes, in spite of the superficial similarities. Additionally, not everyone who uses psychedelics experiences synaesthesia; perhaps only those with a genetic predisposition will experience synaesthesia under the influence of psychoactive drugs. In this paper we have four major goals. First, we will review some recent experiments we have done which establish clearly, for the first time, that synaesthesia is genuinely sensory (Hubbard & Ramachandran, 2001; Ramachandran & Hubbard, 2000; 2001a). Second, we will consider a number of seemingly unrelated facts about synaesthesia and certain other neurological disorders and link these into a coherent new theoretical perspective. Third, we will discuss the relevance of this scheme to certain enigmatic aspects of human nature such as metaphorical thinking, art and the origin of language. And fourth, we will use this theoretical framework to make several new experimental predictions about both synaesthesia and other, more elusive, aspects of the mind. The facts we propose to link together are the following:1 1) 2) 3) 4) Synaesthesia runs in families. Synaesthetes often report ‘odd’ or weird colours they cannot see in the real world but see only in association with numbers. We even saw a colour-blind subject recently who saw certain colours only upon seeing numbers. If a person has one type of synaesthesia, she is also more likely to have a second or third type. There appears to be tremendous heterogeneity in synaesthesia. We have recently suggested that there may be distinct groups we call ‘higher’ and ‘lower’ synaesthetes that can be operationally defined and distinguished by our experiments. Patients with damage to the left angular gyrus have dyscalculia — they cannot perform even elementary arithmetic. But they can still recognize number graphemes. The angular gyrus is a seat of polymodal convergence of sensory information. Angular gyrus lesions also lead to anomia and, intriguingly, loss of ability to understand metaphors. Ordinary language use is rich with synaesthetic metaphors (‘loud shirt’ or ‘hot babe’). This raises a fascinating question: What is the exact connection — if any — between metaphor and synaesthesia? Synaesthesia appears to be more common among artists, poets, novelists and creative people in general. Why? What is the link? (Unfortunately, this

5) 6) 7) 8) 9)

[1] See text below for references for these points.

for the most part. we will show that they are indeed related. as one student told us. systematic. More recent. They found that synaesthesia is more common in females than males (6:1) and that approximately one-third of their respondents had known family members who were also synaesthetic.000 people (Baron-Cohen et al. Baron-Cohen et al.6
V. More recently. For example. Cytowic (1989. HUBBARD
higher prevalence among artists fuels the view that the phenomenon is just the result of ‘craziness’ or vague metaphorical speech because. It seems disproportionately aversive. empirically testable. 1997) estimates that it occurs in 1 in 20. unpublished observations).) 10) Synaesthetes often report that if the number is printed in the wrong colour ‘it looks ugly’. given the existence of different types of
synaesthesia (possibly with different genes being involved). many of his subjects had relatives who were also synaesthetic. 2001). ‘artists are all crazy anyway’. RAMACHANDRAN & E. 1996). But in some synaesthetes. but some of this might also be due to the different subtypes examined by different investigators. The genetics might become clearer once the different phenotypes have been more clearly characterized using our psychophysical probes. the colour is more vivid than when it is actually seen (presumably not being vetoed by actual sensory input). Our own results indicate that the prevalence may be even greater. perhaps as much as 1 in 200 (Ramachandran et al. The ideas we will present are very speculative but have the advantage of being. Cytowic focussed on taste–shape synaesthesia.S.000 people. Also. and surprisingly.2
[2] The patterns of inheritance are likely to be complicated. and that it may be dominant (Bailey & Johnson. Some of this variability is probably due to differences in definitional criteria used by different researchers. In spite of the variability in estimates of its prevalence. while Galton (1880) placed the prevalence at 1 in 20. 1997). which is the most common subtype (Day. perhaps not all cases of grapheme–colour synaesthesia have a genetic basis. while we focus on grapheme–colour synaesthesia. studies have estimated that synaesthesia occurs in 1 in 2. and that our research into synaesthesia has the potential to illuminate a number of more elusive aspects of the human mind. Why? 12) Most synaesthetes claim that even when they visualize the number.
. almost every study of synaesthesia has agreed that synaesthesia seems to run in families.M. Why do they have such a violent emotional reaction to such a trifling discord? 11) There are hints that patients with TLE may have a higher incidence of synaesthesia. some may have an epigenetic cause. the corresponding colour is seen. While this collection of facts may seem at first to be rather arbitrary.. Genetic Basis of Synaesthesia Estimates of the prevalence of synaesthesia vary dramatically. Family studies show that the trait seems to be passed along the X-chromosome.. (1996) conducted a more formal survey to determine the familiality of synaesthesia. the colour associated with imagined numbers is actually less vivid. Galton (1880) first noticed this.

Our two synaesthetes. After all.6% consistent. all of which suggest that grapheme–colour synaesthesia is a sensory effect rather than a cognitive one or based on memory associations (Ramachandran & Hubbard. We measured their performance and found that they were significantly better at detecting the embedded shape than non-synaesthetic control subjects (Ramachandran & Hubbard. Is Synaesthesia Perceptual or Cognitive? We conducted five experiments3 with our first two synaesthetes (JC and ER). while control subjects were only 37. our first two subjects replied with remarks like. May and June will all be green). But I also know it’s just black. We presented subjects with displays composed of graphemes (e. They don’t blend (yet another bit of evidence against the memory hypothesis). THOUGHT AND LANGUAGE
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Synaesthetic Associations are Stable over Time Synaesthetically induced colours are consistent across months or even years of testing (Baron-Cohen et al. that’s hard to answer.g. non-synaesthetic subjects find it hard to detect the embedded shape composed of ‘5’s. we embedded a shape — such as a triangle — composed of other graphemes (e. Sometimes clusters of adjacent days or months will have very similar colours (e. 2001a). synaesthetes often report hybrid colours on the number. computer-generated ‘2’s). Control subjects were told that they would be tested one week later. While this proves that the effect is not confabulatory in origin.SYNAESTHESIA — PERCEPTION.g. April. even without external reinforcement. Another intriguing observation is that some synaesthetes experience colours only for numbers but not letters.. back cover). making it clear that they were not confabulating and could not have been ‘faking it’. a matrix of randomly placed.. Since ‘5’s are mirror images of ‘2’s and made up of identical features (horizontal and vertical line segments). the colours are splotchy as in a Dalmatian dog. It’s not a memory thing. 1. Baron-Cohen et al. But with the banana I can also imagine it to be a different colour. such as ‘reddish blue — but not purple’ or ‘whitish green’. 2000. I do see the colour red. even though this strategy is unpopular in conventional psychophysics. on the other hand.3% consistent.
. Within the display.g. Many of these individuals also have a ‘calendar line’ or wheel with each month tinged a particular colour (the same for days of the week). see fig. 2001a). if each number triggers a highly specific memory. It’s hard to do that with the 5’. For instance. Also. while synaesthetic subjects were retested one year later and were not informed prior to testing that they would be retested. 1993). to the question ‘Do you literally see the number “5” as red or does it merely remind you of red — the way a black-and-white half tone photo of a banana reminds you of yellow?’. 1) Synaesthetically induced colours can lead to pop-out. providing plenty of opportunity for rehearsal. Perceptual grouping and pop-out are often used as diagnostic tests
[3] Common sense suggests that it would also be worth probing the introspective phenomenological
reports of these subjects. 2001a). it does not necessarily show that it is sensory rather than conceptual or based on early memories. Synaesthetic subjects were 92. this might have been remembered by the subject with each occurrence of the number over a lifetime. all of which further suggests that synaesthesia is a sensory phenomenon (Ramachandran & Hubbard. computer-generated ‘5’s. see the ‘2’s as one colour and the ‘5’s as a different colour. asked nine synaesthetic subjects and nine controls to give colour associations for a list of 130 words. ‘Well.. so they claim to see the display as (for example) a red triangle amidst a background of green ‘2’s.

suggesting that it is the visual grapheme.’ This observation implies. 2001). We have found that when a number was moved beyond 11 degrees into peripheral vision and scaled for eccentricity (Anstis. i. the latter is a high-level linguistic concept.S. that triggers the perception of colour. 2001a). the induced colours are genuinely sensory in nature. 3. We optically superposed two different graphemes and alternated them. these five sets of experiments prove conclusively that. 2. tilted lines can be grouped and segregated from a background of vertical lines but printed words cannot be segregated from nonsense words or even mirror reversed words. back cover).4 The question is. HUBBARD
2)
3)
4)
5)
to determine whether a given feature is genuinely perceptual or not (Beck. 1966. 1998). Individual graphemes presented in the periphery are easily identified. Subjects experienced colours alternating up to 4 Hz. it lost its colour (Ramachandran & Hubbard. We have found that the crowded grapheme nevertheless evoked the appropriate colour. again. RAMACHANDRAN & E.
. 1970. even though it was still clearly visible (fig. when other letters flank the target.8
V. The subject said.. 2000. We have found that even ‘invisible graphemes’ can induce synaesthetic colours. see Harrison & Baron[4] This conclusion also receives confirmation from a recent study which showed that a grapheme that
evokes a synaesthetic colour is more difficult to detect against a background of identical colour (Smilek et al. the converse of the pop-out effect we have previously described. He et al. 1982). that the colour is evoked at an early sensory — indeed preconscious — level rather than at a higher cognitive level. back cover). In fact. This effect (‘crowding’) is not due to the low visual acuity in the periphery (Bouma.e. At higher speeds — up to 10 Hz — the numbers could still be seen alternating but our subjects said they no longer experienced colours (Ramachandran & Hubbard. not the numerical concept. In a third subject the colours started alternating at a much slower rate of once every two or three seconds (as in binocular rivalry). what causes the condition? The Cross-Activation Hypothesis The idea that synaesthesia may be the result of some form of cross-wiring has been around for at least 100 years (for lucid reviews. a curious new form of blindsight (Hubbard & Ramachandran. Thus. For example. the target grapheme is large enough to be resolved clearly and can be readily identified if the flankers are not present. Treisman. Tactile and auditory letters were also ineffective in evoking colours unless the subject visualized the grapheme (Ramachandran & Hubbard. Roman numerals and subitizable clusters of dots were ineffective in eliciting synaesthetic colours. 2000. The former is a perceptual difference in orientation signalled early in visual processing by cells in area V1. However. 2001b). 1996).M.. 2000. 2001.
Taken collectively. not the numerical concept that is critical (but see below for exceptions). it is the visual grapheme. it is difficult to identify the target grapheme (fig. 2001a). 2001a). ‘I can’t see that middle letter but it must be an “O” because it looks blue.. at least in some synaesthetes. Ramachandran & Hubbard.

Consequently. 1995. in a manner analogous to the cross-activation of the hand area by the face in amputees with phantom arms (Ramachandran et al. but not in controls.. why doesn’t the cross-wiring
also occur between face cells and colour? To be sure. Hadjikhani et al. If so.. To date. 4. Nobre et al. the failure to find activity in early visual areas (e. there has been only one imaging study. 1997. remarkably. Paulesu et al. no precise anatomical localization was possible given the limits of resolution of the technique. mainly to this particular form of synaesthesia. 2001a) but we don’t know how common this may be. it is usually stated in very vague terms and anatomical localization has not been properly investigated.SYNAESTHESIA — PERCEPTION. Can it be a coincidence that the most common form of synaesthesia involves graphemes and colours and the brain areas corresponding to these are right next to each other? We propose. especially in the left hemisphere (Tarkiainen et al. We were struck by the fact that. Lueck et al. physiological and imaging studies in both humans and monkeys. Marks. Ramachandran & Hirstein. Regional cerebral blood flow (rCBF) measurements were taken during tone listening and word listening.. 1998).
. V2 or V4 — were activated significantly more during word listening than during tone listening in synaesthetic subjects. However. every time there is activation of neurons representing numbers. we have encountered at least one synaesthete who saw faces tinged with colour that was modulated by facial expression (Ramachandran & Hubbard. The key insight comes from anatomical. Zeki & Marini. 1997). (2001). One possibility is that ‘face nodes’ are represented in too complex a manner for a simple one-to-one cross-activation of colour to occur. We will therefore confine our speculations.. number– colour or letter–colour) synaesthesia. adjacent to V4 (fig.g. 1992. although we believe the argument may be valid for other kinds as well. THOUGHT AND LANGUAGE
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Cohen. In the immature brain. back cover). 1999). there may be a corresponding activation of colour neurons. 1998).6 One potential mechanism for this would be the observed prenatal connections between inferior temporal regions and area V4 (Kennedy et al. 1989. [6] The fusiform gyrus also has cells specialized for recognizing faces.. 1998 and V8. 1998) are in the fusiform gyrus. 1994.. in this paper. (1995) report a PET study in which word–colour synaesthetes were presented with pure tones or single words. In addition. as opposed to a true absence of activity (Gray. However. 1997). Areas of the posterior inferior temporal cortex and parieto–occipital junction — but not early visual areas such as V1. there are substantially more connections between (and within) areas than are present in the adult brain.. Some of these
[5] A similar idea has more recently been proposed by Smilek et al. The most common type of synaesthesia is grapheme–colour (i. 1994. 1997. 2000). the visual grapheme area is also in the fusiform (Allison et al. therefore. Our goal here will be to make some concrete testable proposals regarding the exact anatomical locus (or loci) and the extent of ‘cross-wiring’ and to link this idea with the long list of other seemingly unrelated facts listed above. Pesenti et al... which show that colour areas in the brain (V4. V4) may also have been due to the limited power of PET. Rodman & Moore. Ramachandran & Rogers-Ramachandran.5 Since synaesthesia runs in families we suggest that a single gene mutation causes an excess of cross-connections or defective pruning of connections between different brain areas. that synaesthesia is caused by cross-wiring between these two areas.e..

Hence.e. Holcombe et al. cortical) phenomena all occur on relatively slow time scales (He & MacLeod. On the other hand. Grossenbacher’s model generally assumes that information is processed up through several levels of the sensory hierarchy to some multi-modal sensory nexus before being fed back to lower areas. This would explain why the connections are not haphazard. An important point to be made here is how our model differs from Grossenbacher’s (1997. 1994. Kennedy. The excess cross-activation merely permits the opportunity for a number to evoke a colour. In the foetal macaque. and that information does not have to go all the way ‘downtown’ before being sent back to colour areas. when presented with a ‘7’ and a ‘3’ (which elicit green and red. 1997. different synaesthetes have different colours evoked by the same numbers). one colour dominated over the other for extended periods.. the macaque homologue of human inferior temporal cortex). since V4 mainly represents central vision (Gattass et al. In a third subject (JC). 2001) model of synaesthesia as a result of disinhibited cortical feedback. if a genetic mutation were to lead to a failure of pruning (or stabilization) of these prenatal pathways. RAMACHANDRAN & E. JC might see red dominate for a period of several seconds. connections between the number grapheme area and V4 would persist into adulthood. indeed. In our model (at least for grapheme–colour synaesthesia). at approximately 6 Hz. For example. A second important point is that. The cross-activation hypothesis can explain our finding that in some synaesthetes the colours are evoked only in central vision.10
V. There may be internal developmental or learning rules that dictate that once a connection has formed between a given ‘number node’ and a ‘colour node’. and others remain.. It has been shown that there is a much larger feedback input from inferior temporal areas to V4 in prenatal monkeys. while in the adult. no further connections can form.. leading to the experience of colour when viewing numbers or letters. 1995. Grossenbacher & Lovelace. Recent evidence shows that the time course of central perceptual (i. then green. even though he could clearly see
. Since memories ordinarily show positional invariance our observations imply that synaesthesia is not just associative memory from childhood. A given number only evokes a single colour.. we propose that the connections are much more local. approximately 20–30% of retrograde-labelled connections to V4 come from higher areas (Kennedy et al. personal communication). He et al. The cross-wiring hypothesis is also consistent with the finding that synaesthetic colours were no longer experienced in two of our subjects when two spatially overlapping numbers were temporally alternated at rates exceeding 6 Hz. approximately 70–90% of the connections are from higher areas (especially TEO.S.M. 1988. respectively). Rosa 1997). HUBBARD
connections are removed through a process of pruning. the final expression must require learning — obviously one isn’t born with number and letter graphemes hardwired in the brain (and.. even though the alternating numbers could still be clearly seen. even though we postulate a genetic mutation that causes defective pruning or stabilization of connections between brain maps. if the cross-wiring occurred disproportionately for central vision then one would expect the colours to be evoked selectively in this region. 2001). such as V4.

p. our model may explain why people who experience one kind of synaesthesia (e. he no longer experienced colours in response to musical tones.. after the accident. this fusiform activity is sufficient to evoke colours in parallel (which are not affected by crowding). One final piece of evidence for the ‘hyperconnectivity’ hypothesis comes from cases of acquired (as opposed to hereditary) synaesthesia. in which letters and numbers ‘looked like Greek or Hebrew to him’ (Sacks & Wasserman. see a recent fMRI study by Dehaene et al. However. Prior to the accident. for example. only the actual Arabic numerals evoke colours — Roman numerals and subitizable clusters of dots do not. and therefore subsequent colour selective regions are also activated. he started to experience tactile sensations as visual phosphenes.. 1999). This observation suggests that it is the actual visual appearance of the grapheme. We suggested
. grapheme–colour) are more likely to experience another (e. it may be expressed in a patchy manner to different extents and in different anatomical loci in different synaesthetes. However. Perhaps this brain region was also critical for his synaesthesia (see below).. 2001). Interestingly. 1988) report a patient who became colour blind (cerebral achromatopsia) after his car was hit by a truck. the patient was an artist who also experienced colours when presented with musical tones. and so when it was damaged he no longer experienced synaesthesia. leading to both the loss of colour vision and the acute alexia. neuronal activity in the fusiform is necessary. our hypothesis may shed light on the neural basis of other forms of synaesthesia. consistent with what is now known from the imaging literature. The failure of pruning might occur at multiple sites in some people. in JC and ER.g. the processing of the graphemes does not extend beyond fusiform. our hypothesis can also explain why a number rendered invisible through crowding can nevertheless evoke colours. This leads to our next postulate: Even though a single gene might be involved. This is consistent with cross-activation in the fusiform because the latter structure represents the graphemes. Sacks et al. 1987.g.. Fourth. This would indicate that a single brain region might have been damaged in the accident.SYNAESTHESIA — PERCEPTION. Perhaps due to crowding. a few years later. Third. Perhaps perceptual events do not reach consciousness in the fusiform — the place where we postulate the cross-activation to be occurring (i. This ‘rivalrous’ phenomenon is difficult to explain on a memory or metaphor account of synaesthesia. THOUGHT AND LANGUAGE
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the numbers alternating at a higher frequency. not the concept. tone–colour). the subject also reported acute alexia. Sacks and Wasserman (1987.. Remarkably. This may depend on the expression of certain modulators or transcription factors. We recently examined a patient who had retinitis pigmentosa and became progressively blind starting from childhood until he became completely blind at 40. not the numerical concept that evokes colour. Finally. leading to the conscious experience of the colours.e. but not sufficient for conscious awareness. 26). The idea also explains why. The tactile thresholds for evoking the phosphenes (‘synaesthesia threshold’) were higher than the tactile thresholds themselves and the synaesthesia thresholds were constant across intervals separated by weeks — implying that the effect is genuine and not confabulatory in origin (Armel & Ramachandran.

1993).12
V.
. Bearing in mind the enormous number of reciprocal connections even between visual areas that are widely separated (e.S. augmented by ERPs should help to resolve the issue. see fig. the way in which it is classified is important in determining which colour is actually evoked.. when we show our synaesthetic subjects a display like ‘THE CAT’ (see fig. alternating between seeing (say) red and green (Ramachandran. 1996. either through an excess of anatomical connections or defective pruning.g. However. 1997). JC and ER can voluntarily switch back and forth between seeing the ‘forest’ and the ‘trees’. there may be merely a failure of inhibition between adjacent regions causing leakage between areas that are normally insulated from each other (Baron-Cohen et al.M. (2) disinhibition between adjacent areas. preparation). Synaesthesia: Cross-Wiring or Disinhibition? The cross-activation of brain maps that we postulate can come about by four different mechanisms: (1) cross-wiring between adjacent areas. anatomically close maps are also often more likely to be cross-wired at birth. one could reasonably conclude that the hereditary condition might also have a similar neural basis. Furthermore. RAMACHANDRAN & E. it can be modulated by top-down influences. the strengthening (or failure of developmental pruning) of any of these connections could lead to cross-activation of brain maps that represent different features of the environment. we have shown that. Second. 2000a).. either the back-projections linking these areas become hyperactive or new pathways emerge. Top-Down Influences in Synaesthesia Although we have so far focussed on cross-wiring and early sensory effects. 1995). such as attention. 1997). instead of the creation of an actual excess of anatomical connections. (3) increased feedback connections between successive stages of the sensory hierarchy and (4) excess activity between successive stages in the hierarchy as a result of disinhibition of feedback connections. First. thereby providing greater opportunity for the enhanced cross-wiring that might underlie synaesthesia (Kennedy et al.. Future imaging studies (Hubbard et al. Felleman & Van Essen. We have conducted several experiments to demonstrate such effects. HUBBARD
that the visual deprivation causes tactile input to start activating visual areas. although the visual form is necessary for the perception of the colours.. it does not follow that synaesthesia cannot be affected by top-down influences. 1991. 6) they report that they see the correct colour for the ‘H’ and the ‘A’ immediately. indeed the more neutral phrases ‘cross-activation’ or ‘crosstalk’ might be preferable. except that the cause is genetic rather than environmental. when presented with a hierarchical figure (say a ‘5’ composed of ‘3’s. We use the expression ‘cross-wiring’ somewhat loosely. This shows that although the phenomenon is sensory. Kaas. since the length of neural connections tends to be conserved developmentally (Johnson & Vecera. even though the two forms are identical. Hence. 5). If such cross-activation (based on hyperconnectivity) is the basis of acquired synaesthesia. Van Essen & De Yoe.

Taken together. However. we presented the sentence ‘Finished files are the result of years of scientific study combined with the experience of years’ and other such examples of unitization (Goldstone.
Figure 6. they report the colour switching to the one they see for a 3.SYNAESTHESIA — PERCEPTION. our synaesthetes reported that they spontaneously saw the colours switch back and forth as the percept switched back and forth between the Roman numerals and letters. Left: When presented with the ambiguous H/A form in THE CAT. this should not be taken to imply that grapheme–colour synaesthesia is a conceptual phenomenon. Ambiguous stimuli demonstrating further top-down influences in synaesthesia. Hierarchical figure demonstrating top-down influences in synaesthesia. 1994). implying that the unitization constrains the emergence of the synaesthetic colour. like many other perceptual phenomena such as the famous Rubin face–vase or the Dalmatian.. even though the physical stimulus was identical in both cases. Non-synaesthetes usually detect only three — they are ‘blind’ to ‘F’ in the three ‘of’s because words such as ‘of’ come to be treated as single lexical units. he reported seeing the colour appropriate to letters when the display was perceived as letters. LaBerge & Samuels.
. both of our synaesthetes reported that they experienced different colours for the H and the A. Right: When presented with a display of this sort. our synaesthete said that she initially saw only three ‘red graphemes’ in the sentence. it merely indicates that. when we presented another synaesthete with a display that could either be seen as the letters ‘I’ and ‘V’ or as the Roman numeral four (fig. Instead. Likewise. that could be seen as either the Roman numeral 4 or the letters IV. 6). when they shift their attention to the 3s that make up the 5. THOUGHT AND LANGUAGE
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Figure 5.
Third. these experiments demonstrate that synaesthesia can also be strongly modulated by top-down influences. When our synaesthetic subjects attend to the global 5. but on careful scrutiny saw all six ‘F’s tinged red. but not when it was perceived as the numeral four. they report the colour appropriate for viewing a 5.
Unitization Affects Synaesthesia To further explore the idea that top-down influences affect synaesthesia. 2000. However. Whether the result would be different for lower and higher synaesthetes (see below) remains to be seen. 1974) to one of our higher synaesthetes (RT) and asked her to count the number of ‘F’s in it. cognitive influences can also influence early sensory processing (Churchland et al.

1940. Craver-Lemley & Reeves.S. in some synaesthetes. HUBBARD
Synaesthesia and Visual Imagery We find that most synaesthetes report that when they imagine the numbers. Intriguingly. More recent work has clearly shown that this is not a simple criterion shift. It has been known from several decades of clinical neurology that this region is concerned with abstract numerical calculation. 2000) and early visual pathways (Farah. a real image of the object was back-projected on the screen and gradually made brighter. subjects looked at a translucent white screen and were asked to imagine common objects. RAMACHANDRAN & E. 2000. but rather. Higher and Lower Synaesthetes The findings we have discussed so far were true for the first two synaesthetes we tested: They both saw colours only in central vision and only with Arabic numbers.14
V. Segal. 1971). 1995). In them it is the concept of numerical magnitude that seemed to generate colours. it is this higher or more abstract number area that is cross-wired to the colour area? A suitable candidate is the angular gyrus in the left hemisphere.. would synaesthetes find it harder to detect this than non-synaesthetes? And would the outcome be different for higher and lower synaesthetes? As a control one could measure the threshold for detecting the ‘wrong’ colour. 1999. Dehaene. However. say green. damage to it causes acalculia (Gerstmann. Remarkably. The patient cannot do even simple
. 1992. 1992)... An analogous experiment can be performed on synaesthetic subjects. introduced into the number. 1992. This may be because engaging in mental imagery partially activates both category-specific regions involved in visual recognition (O’Craven & Kanwisher. One could show them a red number on a white background at varying contrasts. although we have seen occasional exceptions to this rule. 2000. Kosslyn et al. If we then test synaesthetes and non-synaesthetes to determine the lowest contrast (or saturation) at which they can detect the number. the extent and locus of top-down partial activation. In Perky’s original experiment. there may be varying degrees of synaesthesia induced by imagery in different subjects. 1997). the corresponding colours are evoked more strongly than by actual numbers. is a true reduction in perceptual sensitivity (Craver-Lemley & Reeves. we have subsequently encountered other synaesthetes in whom even the Roman numeral or a cluster of dots elicited the colour. Depending on the locus of cross-wiring (whether they are higher. Farah et al. even days of the week or months of the year were coloured. lower or mixed synaesthetes). 1910. subjects failed to report the presence of the image. Using a slide projector. and the extent to which this is vetoed by real bottom-up activation from the retina.M. Klein et al. it was clearly above threshold). Grewel. Could it be that there is a brain region that encodes the abstract numerical sequence or cardinality — in whatever form — and perhaps in these synaesthetes. 1952. even though the projected object would have been clearly visible under normal circumstances (that is. an effect that provides an elegant technique for probing the elusive interface between imagery and perception. We are currently investigating these issues using the ‘Perky effect’ (Perky.

who experienced synaesthesia prior to the injury. One clear prediction is that the psychophysical properties of the colours evoked in higher synaesthetes might be different from those of lower synaesthetes. ‘shiny’ or ‘patchy’ compared to the upper case letter. days of the week and letters of the alphabet. Spalding & Zangwill (1950) report a patient with a gunshot wound. in JC. while ours is specifically based on possible neuroanatomical divisions. the level of cross-wiring would be different so that one could loosely speak of ‘higher’ synaesthetes and ‘lower’ synaesthetes. It is tempting to postulate that these two regions — the higher colour area and the abstract numerical computation area are cross-wired in some synaesthetes. we recently encountered an exception. 1998). Remarkably. Our view also differs from Martino and Marks’ in that they do not consider the anatomy. We believe that both higher and lower synaesthesia represent a form of strong synaesthesia. we usually find that upper and lower case letters evoke the same colour.. Until we know more about the neural underpinnings of synaesthesia we must be prepared for some surprises. THOUGHT AND LANGUAGE
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arithmetic such as multiplication or subtraction.g. his forms for months. days of the week). and the lower case letters were usually less saturated. In addition. even the actual font seemed to matter. runs of colours corresponding to numerical sequences match up at least partially with runs corresponding to weeks or months. For instance. e was red. the subsequent colour areas in the cortical colour-processing hierarchy lie in the superior temporal gyrus. in these higher synaesthetes. complained that his ‘number plan’.g.. (However. In view of this. Five years after injury he complained of spatial problems and showed difficulty in number tasks. but involve different hierarchical levels and have different neuroanatomical correlates. whereas a second subject reported that the silent /p/ lost its colour when he was reading fast but not when he read slowly.8 Another such difference is that
[7] Our distinction between higher and lower synaesthesia is orthogonal to Martino and Marks’ distinc-
tion (2001) between ‘strong’ and ‘weak’ synaesthesia. subject RT said ‘When you roll the dice. Japanese and English). the fact that it was silent didn’t matter. this was the case. which entered near the right angular gyrus and lodged near the left temporal– parietal junction. was no longer distinct. If a synaesthete is bilingual (e. Subitizable dot clusters are usually seen as not coloured (although this might depend on whether the subject is a higher or lower synaesthete). would visually dissimilar graphemes that represent the same phoneme (in Japanese and English characters) evoke the same or different colours? We have encountered both types. I know it’s a six or a four but I don’t see colours. For example. different fonts would evoke the same approximate colour but slightly different hue values.SYNAESTHESIA — PERCEPTION..) Also. Furthermore. Indeed. the answer seems to depend on whether one is dealing with higher or lower synaesthetes. as with the ‘p’ in ‘psychology’)? Is the ‘p’ tinged the same colour in psychology as in Pat? In JC.7 Perhaps the angular gyrus represents the abstract concept of numerical sequence or cardinality and this would explain why in some higher synaesthetes even days of the week or months of the year elicit colours. [8] Several intriguing new questions are raised by these speculations: What if the grapheme is embedded in a word but is silent (e. It would be interesting to test Japanese-American synaesthetes to see if /r/ and /l/ evoke the same colour or not given that they sound the
. But. the patient. it might be interesting to see if patients with angular gyrus lesions have difficulty with tasks involving sequence judgment (e. In subject SP most letters followed the rule but one letter had completely different colours for upper and lower case: E was green. For example. Consistent with this hypothesis. depending on the level at which the gene is expressed (fusiform or angular).g. adjacent to angular gyrus (Zeki & Marini. in higher synaesthetes the colours might not fall off with eccentricity and/or might not be produced if crowding masks the grapheme. since different cortical colour areas are involved. if I say “Hey I got a five” and I imagine five then the colour is seen’. it would be interesting to see if.

In some synaesthetes.g. RAMACHANDRAN & E.S. other than the subjects’ introspective reports. the reaction time (RT) is not the same. Finally. with the distance decreasing between numbers as the numerical magnitude increases. the number line is a series of segments going upwards and rightwards. if different graphemes in a word evoke similar colours. so that if all the number locations were mapped out they formed a continuous line with no breaks or jumps.M. Numbers that were close (e. if the convoluted line doubles back on itself.16
V. and therefore may not demonstrate the effect predicted above. or. Lastly. 2 and 4) were close spatially in this imaginary space.. In the other. consider the so-called ‘number line’ (Dehaene et al. If so. ‘2 or 4?’. and does not extend into the negative numbers.g. remarkably. the colour of the first grapheme seems to dominate: It influences the perceived colour of the entire word (or even non-word). Also. they take longer in the first case than in the second. the colours tend to enhance each other. Additionally. since lower synaesthetes may not experience the convoluted number line. Surely. the number line is curved. [9] In one of our subjects. Galton (1880) found that some otherwise normal people claimed that when they visualized numbers. even though the line was continuous. whereas if they evoke dissimilar colours the colours tend to weaken each other (especially when they are close together). 1990) seen in normal subjects. there has not been a single study that demonstrates objectively that these subjects really do have a complex convoluted number line. To demonstrate this we propose the following experiment. observations such as these may eventually give us valuable insights into the manner in which neurons in the human brain represent knowledge (we have dubbed this new discipline ‘neuro-epistemology’).9 Unfortunately. it was not straight. This experiment may also provide an experimental method to differentiate higher and lower synaesthetes. It has been suggested that there is an analogue representation of numbers in the brains — a ‘number line’ which can be read off from to determine which number is bigger — and obviously it will be more difficult if the numbers are closer on this line (Dehaene. It was often highly convoluted. we have also noticed some intriguing spatial interaction effects between synaesthetically induced colours. each number always occupied a specific location in space. 2 and 15 may be closer to each other ‘as the crow flies’ but farther along the number line itself)? Such a result would provide objective evidence for the subjective reports of these synaesthetes in a manner analogous to the texture segregation and crowding experiments for grapheme–colour synaesthetes described above. If we ask subjects which of two numbers is bigger. 1997).. The number line continues left of fixation to represent negative numbers and its shape is a mirror reflection — on both axes — of the number line for positive numbers. 1985). This would be consistent with data supporting the idea that attentional gating is more effective at higher levels of the visual hierarchy (Moran & Desimone. even curving back on itself. ‘2 or 15?’. One could give them two numbers that are numerically far apart (e. We have two subjects with this condition in our sample of synaesthetes.
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higher synaesthetes may require more focussed attention in order to experience their colours. the graphemes on the number line are are seen to be rotated so that they remain orthogonal to the line.. 2 and 15) but spatially close together because of the convoluted number line.g.
same to them. However. would the reaction time for responding which number is bigger depend on proximity in abstract number space or would the subject take a ‘short-cut’ (e.

if this mutation is more diffusely expressed it may produce a more generally cross-wired brain creating a greater propensity and opportunity for creatively mapping from one concept to another (and if the hyperconnectivity also involves
. Perhaps many other concepts are also represented in non-topographic maps in the brain. 1999). ‘Juliet is the sun. Ironically it is this heterogeneity that has often caused researchers to avoid studying synaesthesia altogether or led them to conclude that the whole phenomenon is bogus. Does that mean she is a glowing ball of fire?’ (Schizophrenics might say this. How is this achieved? It has often been suggested that concepts are represented in brain maps in the same way that percepts (like colours or faces) are. 1989. suggesting that synaesthesia may be more common among fine-arts students than the population at large. One such example is the concept of number. 1997. 84 (23%) reported experiencing synaesthesia. Instead. we can think of metaphors as involving cross-activation of conceptual maps in a manner analogous to cross-activation of perceptual maps in synaesthesia. they often interpret metaphors literally). we should note that it is possible that the distribution of gene expression and level of cross-activation is not bimodal. For example.SYNAESTHESIA — PERCEPTION. it suffers a severe limitation in that no experimental tests were conducted to assess synaesthetic experiences. If mutation-induced cross-wiring selectively affects the fusiform or angular gyrus someone may experience synaesthesia. How can the cross-wiring hypothesis explain these results? One thing these groups of people have in common is a remarkable facility linking two seemingly unrelated realms in order to highlight a hidden deep similarity (Root-Bernstein & Root-Bernstein. year in school and verbal intelligence) on four experimental measures of creativity. Artists. Poets and Synaesthesia Synaesthesia is purported to be more common in artists. ‘She is warm like the sun. He found that. Domino (1989) reports that. Indeed. Further studies making use of our objective experimental measures of synaesthesia are clearly required to confirm this result. major. nurturing like the sun. synaesthetes performed better than controls on all four experimental measures of creativity. a fairly abstract concept. However. yet we know that specific brain regions (the fusiform and the angular) are involved. THOUGHT AND LANGUAGE
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Finally.. hence the heterogeneity of the phenomenon. While this study has the advantage of using an experimental method to assess creativity. our brains instantly understand this. as a group. 1999). Domino. Domino then tested 61 of the self-reported synaesthetes and 61 control subjects (equated on gender. You don’t say. one might expect to encounter ‘mixed’ types rather than just higher and lower. If so. in a sample of 358 fine-arts students. your brain instantly forms the right links. radiant like the sun’ and so on. Root-Bernstein & Root-Bernstein. This incidence is higher than any reported in the literature (see above). When Shakespeare writes ‘It is the East and Juliet is the sun’. If this idea is correct then it might explain the higher incidence of synaesthesia in artists and poets. poets and novelists (Dailey et al.

The reason it may seem slightly implausible at first is because of the apparent arbitrariness of metaphorical associations (e. Evolution of Language One of the oldest puzzles in psychology is the question of how language evolved. (Indeed.10 The Angular Gyrus and Synaesthetic Metaphors In addition to its role in abstract numerical cognition. 1998.M. Brownell et al. from the auditory to the visual modality).. ‘loud shirt’) also respect the directionality seen in synaesthesia (Day. We suggest that these rules are a result of strong anatomical constraints that permit certain types of cross-activation. The problem is that several interlocking pieces needed to co-evolve. we have noticed that synaesthetic metaphors (e.) It is even possible that the angular gyrus was originally involved only in cross-modal metaphor but the same machinery was then co-opted during evolution for other kinds of metaphor as well.S.. Furthermore. A large number of metaphors refer to the body and many more are inter-sensory (or synaesthetic). no satisfactory explanation has yet been given for this deficit.g. metaphors are not arbitrary. Lakoff and Johnson (1980) have systematically documented the non-arbitrary way in which metaphors are structured. RAMACHANDRAN & E. but only a gifted few (the ones with more cross-wired brains in our scheme) can be creative in generating them. we would argue that the pivotal role of the angular gyrus in forming cross-modal associations is perfectly consistent with our suggestion that it is also involved in metaphors — especially cross-modal metaphors. but not others. Intriguingly. We realize that this is an unashamedly phrenological view of metaphor and synaesthesia.. and how they in turn structure thought. ‘a rolling stone gathers no moss’). 1975).. unlike normals. Ullman. That is. we recently saw an anomic aphasic with left angular gyrus damage who. Our idea that excess cross-wiring might explain the penchant for metaphors among artists and poets is also consistent with data suggesting that there may be a larger number of cross connections in specific regions of the right hemisphere (Scheibel et al. 1976). Williams.g. they are more frequent one direction than the other (e. 1996.. which we would interpret as a difficulty with metaphor. 1990). HUBBARD
sensory-to-limbic connections the reward value of such mappings would also be higher among synaesthetes). However.g. and the observed role of the right hemisphere in processing non-literal aspects of language (Anaki et al. But how could this have happened given that evolution has no foresight? Alfred Russell Wallace was so frustrated in trying to answer this that he felt compelled to invoke
[10] This argument does not address the important question of why almost anyone can understand a meta-
phor once it is spelled out. patients with lesions here tend to be literal minded (Gardner.. showed no propensity for the bouba/kiki effect described in the next section. 1985). 1945. Yet. parietal and occipital lobes). Based on what we have said so far. the angular gyrus has long been known to be concerned with cross-modal association (which would be consistent with its strategic location at the crossroads between the temporal. Why is such excess cross-wiring needed only for producing metaphors but not for recognizing them?
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V.

consider stimuli like those shown in figure 7. Rizzolatti et al. This means that there would be a natural bias towards mapping certain sound contours onto certain vocalizations. et al. However. The case of ‘suck’.
. Demonstration of kiki and bouba. the founding father of modern linguistics. even though they have never seen these stimuli before. we propose the existence of a kind of sensory-to-motor synaesthesia. which is the actual sound produced when you suck may be an interesting hybrid example. If you show fig. subjects tend to map the name kiki onto the figure on the left.e. 133). These are neurons found in the ventral premotor area in monkeys and (possibly) humans (Altschuler et al.. Fadiga et al.
divine intervention. 95% of people pick the left as kiki and the right as bouba.SYNAESTHESIA — PERCEPTION. More recently. 2000. Köhler (1929) called the stimuli takete and baluma. while the rounded contours of the figure on the right make it more like the rounded auditory inflection of bouba. 1997. Because of the sharp inflection of the visual shape.. 1952). we need to put together several ideas.activation not between two sensory maps but between a sensory (i. for it suggests that there may be natural constraints on the ways in which sounds are mapped on to objects. which may have played a pivotal role in the evolution of language. Marks. even Chomsky. Broca’s area). 1957.. To understand this argument. one of these two figures is a “bouba” and the other is a “kiki”. He later renamed the
baluma stimulus maluma (Köhler. where the rhythm of movements synaesthetically mimics the auditory rhythm. it could not have possibly evolved through natural selection. 7 (left and right) to people and say ‘In Martian language. see Lindauer. First. This somewhat speculative proposal gains credibility from recent work on ‘mirror neurons’ by Rizzolatti and colleagues (di Pellegrino.) Our results again confirm these findings with a different set of stimuli and different names... try to guess which is which’. 1996. (For further discussion. 1947) and further explored by Werner (1934. Werner & Wapner.12 Second. 1992. Our solution to the riddle of language origins comes from synaesthesia. originally developed by Köhler (1929.. as well as the sharp inflection of the tongue on the palate. A familiar example of this is dance.11 The reason is that the sharp changes in visual direction of the lines in the right-hand figure mimics the sharp phonemic inflections of the sound kiki. This type of synaesthesia may be based on cross. The bouba/kiki example provides our first vital clue for understanding the origins of proto-language. 1947). has expressed the view that.
[11] In his original experiments. THOUGHT AND LANGUAGE
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Figure 7. 2001). the results were essentially unchanged and ‘most people answer[ed] without hesitation’ (p. 2000. [12] This idea reminded us of the onomatopoeic theory of language origins (’bow-wow’ = dog) but is quite different in that the relationship between the visual appearance of a dog and the sound made by a dog is completely arbitrary (unlike the kiki/bouba example).e. given the complexity of language. 1990. auditory) and a motor map (i.

1999). and it bears no family resemblance to English. Berlin concludes that this is evidence for universal sound symbolism of the sort we describe here.
. ‘teeny’ and ‘diminutive’ might be synkinetic
[13] With knowledge of these neurons. important factor that may have contributed to this bootstrapping is synaesthesia caused by cross-activation between two motor maps rather than between two sensory maps (a better phrase might be ‘synkinaesia’). words such as ‘little’. HUBBARD
Iacoboni et al. He presented English speakers with fish and bird names from a language completely unrelated to English (Huambisa. But a subset of them.g.g. in our scheme this would be an example of synkinaesia between the motor maps for the mouth and hand. He found that English speakers were able to correctly discriminate bird names from fish names significantly more often than chance. e.. as if to sympathetically mimic the hand movements. pulling something or pushing something).15 A third. In the example cited above. for example.20
V.. which are right next to each other in the Penfield motor homunculus of the pre-central gyrus. Most neurons in this area will fire when the monkey performs complex manual tasks (e. 1966). even though they had never heard Huambisa. [15] This currently quite contentious issue is being studied within linguistics under the banner of ‘phonesthemes’ or sound symbolism (see. 1999) and cross-linguistically (Berlin. words referring to something small often involve making a synaesthetic small /I/ with the lips and a narrowing of the vocal tracts (e. will fire even when the monkey watches another ‘actor’ monkey or human performing the same action. Darwin (1872) noted that when cutting something with a pair of scissors we often unconsciously clench and unclench our jaws.g.S.. 1998). a language of the Jivoran language family in north central Peru). mouth shape for ‘petite’. ‘petite’. 1994). For example. 1999. we conjecture that the representation of certain lip and tongue movements in motor brain maps may be mapped in non-arbitrary ways onto certain sound inflections and phonemic representations in auditory regions and the latter in turn may have non-arbitrary links to an external object’s visual appearance (as in bouba and kiki). Hinton et al. grasping a peanut. highly specific postures (Devereux. Recent research has supported the concept of phonesthemes in English (Hutchins.. mirror neurons. thereby making the origin of proto-language seem much less mysterious than people have assumed (see figure 8). and even the evolution of language (Rizzolatti & Arbib. 1994). imitation learning (Iacoboni et al. We can think of these neurons as doing an internal simulation of such actions. Ramachandran.M. Putting these ideas together. you have the basis for understanding a host of puzzling aspects of
the human mind: ‘mind reading’. 1994). We would also point out that lip and tongue movements and other vocalizations may be synaesthetically linked to objects and events they refer to in closer ways than we usually assume and this may have been especially true early in the evolution of the proto-language of ancestral hominids.13 Another piece of circumstantial evidence for the notion of sensorimotor synaesthesia (and its possible link to mirror neurons) is the occurrence of a rare form of synaesthesia in which sounds evoke the automatic and uncontrollable adoption of certain.. 2000b).14 The stage is then set for a sort of ‘resonance’ or bootstrapping in the co-evolution of these factors. Our new studies on synaesthesia and our speculations on language origins obviously have considerable relevance to this issue of universal sound symbolism. ‘teeny’ and ‘diminutive’) whereas the opposite is true for words denoting large or enormous. RAMACHANDRAN & E. [14] Brent Berlin provides an especially relevant example (Berlin.. After further analyses to rule out onomatopoeia. empathy.

the oral gestures for ‘little’ or ‘diminutive’ or ‘teeny weeny’ synkinetically mimic the small pincer gesture made by opposing thumb and index finger (as opposed to ‘large’ or ‘enormous’). Also. ‘mois’ and ‘I’ mimic pointing inwards towards yourself.g.SYNAESTHESIA — PERCEPTION. the use of tools requires sub-assemblies such as attaching a head to a handle before hammering a nail – and this has the same formal logical structure as hierarchical syntactic tree of language).. semantics and tool manipulation. by mirror neurons). Such synesthetic correspondence could be based on either direct crossactivation or mediated by the angular gyrus – long known to be involved in inter-sensory transformations. (3) Motor to motor mappings (synkinesia) caused by links between hand gestures and tongue. And you pout your lips to say ‘you’.. not that all modern language is synaesthetic in origin. We are suggesting that these factors provided the initial impetus for language evolution. French ‘moi’ and Tamil ‘naan’) In this manner a primitive vocabulary of gesture and pantomime could evolve through synkinaesia into a corresponding vocabulary of tongue/palate/lip movements (causing vocalizations. I also produce a partial outward pout with my lips (as in English ‘you’. ‘vous’ or ‘thoo’ as if to mimic pointing outward whereas ‘me’. If such oral echoes of hand gestures are accompanied by emotional guttural utterances it would lead to the creation of early proto-words. Arrows depict cross-domain remapping of the kind we postulate for synaesthesia in the fusiform gyrus. the flexion of the fingers and palmar crease in ‘come hither’ is mimicked by the manner in which the tongue goes back progressively on the palate). We are currently testing these ideas by studying aphasics. whereas when I point inward to myself. especially if accompanied by guttural utterances).
mimicry of the pincer-like opposition of thumb and forefinger to denote small size. (2) Cross domain mapping (perhaps involving the arcuate fasiculus) between sound contours and motor maps in or close to Broca’s area (mediated. (1) A non. my lips and tongue move inwards (as in English ‘me’. The cross-wiring would necessarily require transforming a map of two dimensional hand gestures into one-dimensional tongue and lip movements (e.g. when pointing I use my index finger to point outward to you. perhaps. The subsequent elaboration and refinement of the deep structure of language may have relied on other environmental selection pressures and biological constraints unrelated to
.g. Notice that each of these effects might be quite small but through progressive mutual bootstrapping they could have evolved into the shared vocabulary of early hominids.. THOUGHT AND LANGUAGE
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Figure 8. and you have fully modern language (e.arbitrary synaesthetic correspondence between visual object shape (as represented in IT and other visual centers) and sound contours represented in the auditory cortex (as in our bouba/kiki example). Add to this additional bootstrapping provided by co-opting the circuits originally used for symbol manipulation. lip and mouth movements in the Penfield motor homunculus (e. A new synaesthetic bootstrapping theory of language origins. French ‘tu’ or ‘vous’ and Tamil ‘thoo’).

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synaesthetic metaphor (and. One wonders. (And then add to this an additional bootstrapping between the syllabic structure. mediated by influences from the Wernicke’s area). a drunk making an unwelcome sexual pass at a woman)? This is unlikely to be coincidence since it is cross-cultural: The Tamil phrase for moral disgust means ‘he smells bad’ and the French word ‘dégoûtant(e)’ (used for social situations) literally means. however. i. whether there exists a genetically based synaesthetic link between sex and aggression (and if so could this have anything to do with the proximity of nuclei concerned with sex with those concerned with aggression in
.) Another example of a ‘synaesthetic metaphor’ found in everyone is the use of the word ‘disgusting’. This is how evolution works. it is yet another example of cross-wiring or even of the same brain map being used for two seemingly unrelated functions (given evolution’s tendency to be opportunistic in using pre-existing hardware). indeed. The key idea here is that each of these different effects (synaesthesia between object appearance and sound contour. 1992) have pointed out that syntactic structure may have arisen from the pre-adaptation provided by syllabic structure. symbol manipulation as well as semantic constraints. Devlin.g. Even the great apes may have some such synaesthetic scatological propensities. the initial emergence of a complex multi-component trait that usually poses a challenge for evolution through natural selection. Chomskyan UG could have evolved more readily. but as mammals became more social it came to communicate or signal olfactory disgust to others (stay away from that rotten food) and then eventually to communicate moral and social disgust (stay away from that rotten man). numerous thinkers (Bickerton. also. ‘bad tasting’. ‘disgusting’. The olfactory bulb projects to the orbito-frontal cortex. and make the same face in response to someone whose behaviour is morally disgusting (e. But why do we use the same word. . When Washoe wanted to ‘sign’ her disgust at someone’s behaviour she used the same word as for faeces (and indeed apes throw faeces at humans whom they are disgusted with). between sound contour and vocalizations. our theory really pertains to the origin of proto-language rather than Chomskyan universal grammar. but a bootstrapping between all of them acting together may have indeed been sufficient. Early mammals may have used the orbito-frontal cortex exclusively for olfactory and gustatory disgust. That is. and synkinaesia) in isolation may have been too small to have exerted adequate selection pressure for the emergence of proto-language. It is.M. Lieberman. and that is what we are trying to explain here. Additionally.S.22
V. RAMACHANDRAN & E. hierarchic. 1995.. and you have fully evolved language. We say this in response to unpleasant smells and tastes while at the same time raising our hands up and scrunching up our noses (Darwin showed that even a newborn infant would do this — suggesting that it is ‘hardwired’). and olfactory and gustatory ‘disgust’ is almost certainly mediated by this part of the frontal lobes. was probably guided by offline. as Francis Crick has said). 2000. but we believe that given the pre-adaptation provided by proto-language.e. given that there is no master design (‘God is a hacker’. symbol manipulation and the syntactic/hierarchical structure. We would argue that this usage emerged because moral and social disgust is also mediated by the orbito-frontal cortex.

If we assume that one’s aesthetic and emotional responses to sensory inputs depend
. mate or predator. depending on how extensively and where in the brain it is expressed (in turn modulated by transcription factors). We have suggested that a mutation that causes hyperconnectivity (either by defective pruning or reduced inhibition) may cause varying degrees and types of synaesthesia. which translates to something like ‘Go F*** yourself’. the use of sexually loaded words as aggressive swear words (‘F*** you’) appears to be cross-cultural. when numbers were the correct colour it ‘felt right. like the “aha” when the solution to a problem finally emerges’. the message gets relayed to the hypothalamic nuclei to prepare the body for fighting. fleeing or mating. prey or mate. one of our synaesthetes claimed that incorrectly coloured numbers were ‘ugly’ and felt like ‘nails scratching on the blackboard’.SYNAESTHESIA — PERCEPTION. a blue carrot or green rose? Anecdotally this seems to be true. LeDoux. Conversely. 1964. In French. patients with temporal lobe epilepsy seem to have a propensity towards synaesthetic experiences. leading one to wonder.. Now imagine what would happen if there were hyperconnectivity between the fusiform gyrus (and other sensory cortices) and the limbic system (especially the amygdala and nucleus accumbens). This autonomic arousal can be measured by monitoring changes in skin conductance caused by sweat — the skin conductance response (SCR) — which provides a direct measure of emotional arousal and limbic activation. If the object is emotionally significant or salient such as a predator. say. If there is no genetic basis related to anatomical/neural constraints. THOUGHT AND LANGUAGE
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the hypothalamus)? Again. but if you look at prey. why do all (or most) languages say ‘F*** you’ and one never hears ‘Bite you’. can we explain it in terms of our cross-wiring or cross-activation hypothesis of synaesthesia? Visual information that is ‘recognized’ by the cortex of the temporal lobe (e. which would be the more logical choice given the obvious semantic associations between biting and aggression? Hyperconnectivity and Emotions Synaesthetes often report strong emotions in response to multi-sensory stimuli (both positive and negative depending on whether the associations are the ‘right’ or ‘wrong’ ones). if you look at neutral objects such as a table or chair there is no arousal or change in SCR. 1992. Why? Despite these subjective reports.. nucleus accumbens and other parts of the limbic system (Amaral et al.g. Neural signals cascade from the limbic structures down the autonomic nervous system to decrease gastric motility and increase heart rate and sweating (e. 1996). Assuming that the claim is true. is their aversion to such stimuli any different from what a non-synaesthete experiences when confronted with. there is. These structures evaluate the significance of the object. Additionally. Lang et al. so that we may speak of the amygdala and nucleus accumbens as developing an ‘emotional salience map’ of objects and events in the world. there is no clear experimental validation of the claim that synaesthetes have strong responses to ‘discordant’ sensory inputs. Typically.g.. 1992). the equivalent phrase is ‘Va t’en faire f**tre’. the fusiform) ordinarily gets relayed to the amygdala. Mangina & Beuzeron-Mangina.

if the seizures (and kindling) were to strengthen the sensory–amygdala connections.M. HUBBARD
on these connections. Jacome.g.. there is no qualia as when you are awake and see a red
[16] There are also anecdotal reports that synaesthesia might be more common among individuals with per-
fect pitch. The repeated seizure activity is likely to produce ‘kindling’ (causing hyperconnectivity between different brain regions) which would explain reports of synaesthesia in TLE (see e. Furthermore. There is now a growing consensus that the best way to solve this ancient philosophical riddle is to narrow down the neural circuitry (Crick & Koch. one could measure the SCR in synaesthetes in response to an incorrectly coloured number and compare this response to one produced in a non-synaesthetic subject who is looking at blue carrots. conversely.. then TLE patients might also be expected to have heightened emotional reactions to specific sensory inputs. 2000) and. We would therefore predict a bigger SCR in the synaesthete looking at the incorrectly coloured grapheme than in control subjects.16 Something along these lines may also explain why some famous artists have had TLE. If our scheme is correct his heightened emotions in response to colours and visual attributes (resulting from kindling) might have indeed fuelled his artistic creativity.g. the reflexive contraction of the pupil in response to light can occur in coma. RAMACHANDRAN & E. by the way. 1990. the functional logic (Ramachandran & Blakeslee.g.. Given that people with perfect pitch have an enlarged auditory representation in the superior temporal gyrus (planum temporale) (Schlaug et al. 1997) of those brain processes that are qualia laden as opposed to those that are not (e. The net result of this will also be a progressive ‘bootstrapping’ of pleasurable or aversive associations through limbic reinforcement of concordant and discordant inputs. Meissner. 1995).24
V. 1998. would produce a disproportionately large emotional aversion (like ‘nails scratching on a blackboard’) and. Metzinger.
. The hyperconnectivity explanation for synaesthesia is also consistent with the claim that the phenomenon is more common among patients with temporal lobe epilepsy (TLE). There are strong hints that this is the case (Ramachandran et al. especially. This explanation reverses the traditional causal arrow that perfect pitch may be more common in people with synaesthesia because the colours allow people to uniquely identify the tones. Synaesthesia and the Philosophical Riddle of Qualia Finally. This. 1997).S. the study of the unusual sensory experiences of synaesthetes may also shed light on the philosophical problem of qualia.. 1995.. Van Gogh being the most famous example (e. allows us to also invoke a form of learning in the genesis of synaesthesia. such as a grapheme in the wrong colour. 1994). Kivalo. A non-synaesthete might be a bit amused or puzzled by the blue carrot but she is unlikely to say it feels like nails scratching on a blackboard. then presenting a discordant input. we would predict that this enlargement may allow hyperconnectivity to occur more readily between auditory and colour maps. however. Ramachandran & Hirstein. In order to test this idea. 1999). harmonious blends of colour and grapheme will be especially pleasant to look at (which may involve the nucleus accumbens rather than the amygdala). producing a higher incidence of sound–colour synaesthesia. 1998.

functional criteria that need to be fulfilled in order for certain neural events to be associated with qualia (a fourth has recently been added. so there is very little qualia (leaving aside the question of whether you can have partial qualia if some criteria alone are fulfilled). as first pointed out by Jeffrey Gray (Gray et al. the output is potentially infinite. The four laws are: Qualia are irrevocable and indubitable. Qualia and attention are closely linked. a paraplegic can even have an erection and ejaculate without an orgasm.. 1998). we would argue that the lower synaesthetes have the qualia of red evoked when they see a ‘5’ or hear C-sharp. Ramachandran and Hirstein (1997) have suggested three ‘laws’ of qualia. This isn’t true for. we have suggested that the critical brain circuits involved in qualia are the ones that lead from sensory input to amygdala to cingulate gyrus (Ramachandran & Hirstein. but using pre-existing. to choose.. Even though the representation at the input level is immutable and automatic. you get just a reflex arc. And lastly. if you have the percept of an apple you can use it to tempt Adam. 1997. or even just to eat. again. stable differences in the conscious experiences of people who experience synaesthesia compared with those who do not. You don’t say ‘maybe it is red but I can visualize it as green if I want to’. 1997. bake a pie. will shed much light on the riddle of qualia. 2) Once the representation is created. A study of circuits involved in attention. this still doesn’t explain why these particular events are qualia laden and others are not (Chalmer’s ‘hard problem’) but at least it narrows the scope of the problem. 1)
. You have the luxury of choice. Gray. therefore. they can be used to shed light on the nature of qualia as well as metaphor (such borderline cases can be valuable in science. Indeed. a spinal reflex arc where the output is also inevitable and automatic. and the study of brain-damaged patients. One strategy used to explore the neural basis of qualia is to hold the physical stimulus constant. THOUGHT AND LANGUAGE
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rose). An explicit neural representation of red is created that invariably and automatically ‘reports’ this to higher brain centres. what can be done with it is open-ended. The input invariably creates a representation that persists in short-term memory — long enough to allow time for choice of output. we are making use of the same strategy. Of course. see Ramachandran & Blakeslee. Without this component. Based on these laws. In the case of synaesthesia.SYNAESTHESIA — PERCEPTION. 4) Attention.g. But when you and I experience red while looking at a black-and-white picture of an apple. while tracking brain changes that co-vary with changes in the conscious percept (e. In particular.. 2001). 1998). You need attention to fulfil criterion number two. Tong & Engel. consider the manner in which viruses helped us understand the chemistry of life). say. e. As such. the red does not fulfil all four criteria specified above. Sheinberg & Logothetis.g. to keep the doctor away. 3) Short-term memory. 1997). the higher synaesthetes may be a borderline case. Synaesthesia — the ‘blending’ of different sensory qualia — obviously has relevance to the qualia problem.

‘When you play me that tone I know it’s a tone and experience it as such but I feel an irresistible urge to say red’ (like a patient with Tourette’s Syndrome). You might say. First. This is yet another piece of evidence against the memory hypothesis — for how can you remember something you have never seen? On the other hand. This may be analogous to
.g. the cross-wiring is unlikely to be very precise. When you are asleep an evil East-coast genius..26
V. Previously.M. 1) If the swapping is done sufficiently early in sensory processing. The cross-wiring hypothesis explains this as well. 2) If the swapping were done at or close to the output stage (e. 2001a). who we’ll call DD. so if you cross wires after the boundary you merely experience an urge whereas if you cross wires before that boundary you literally see red? Is it a fuzzy boundary or a sharp one? We would argue that this boundary corresponds exactly to the point where the transition is made from the four laws of qualia being fulfilled (before the boundary) to where they are not fulfilled (after the boundary). swaps or cross-wires the nerves coming into your brain from your ears and eyes. going to be somewhat different from the spurious or abnormal activation caused indirectly through numbers. the cross-wiring hypothesis explains it neatly. For two reasons. in all likelihood. then the genetically based cross-activation of cells in this area would evoke colour phosphenes even though the colours cannot be seen in the real world because of retinal cone deficiencies. RAMACHANDRAN & E. the activation of cells in the visual centres caused by real world input is. This subject was colour anomalous (s-cone deficiency leading to a difficulty discriminating purples and blues) but intriguingly. given that it is abnormal. Indeed. even synaesthetes who are not colour blind sometimes say that the synaesthetically induced colours are somehow ‘weird’ or ‘alien’ and don’t look quite the same as normal ‘real world’ colours. HUBBARD
To understand the importance of synaesthesia in illuminating the qualia problem consider the following thought experiment performed on your own brain. the outcome is obvious: say the pathways from the auditory nucleus of the brain stem are diverted to the visual cortex and the optic radiations to the auditory cortex. Consider the following places where the wiring could have been swapped. You then wake up.S. he claimed to see numbers in colours that he could never see in the real world (‘Martian colours’). If we assume that the colour processing machinery in V4 in the fusiform is largely innate. But now we come to the key question: What if the swapping or cross-wiring is done at some stage in between these two extremes? Is there a critical boundary between these two extremes. again. Then you would ‘hear’ sights and ‘see’ sounds. Of considerable relevance to this philosophical conundrum is a new observation that we made on a grapheme–colour synaesthete (Ramachandran and Hubbard. It might be slightly messy and this ‘noise’ may be experienced as weird Martian colours. no satisfactory account has been proposed for this. the answer would be obvious. in Broca’s area) where you generate the word ‘red’ or ‘C-sharp’.

Although synaesthesia has been studied for over 100 years. the cross-activation obviously skips the earlier levels of the colour-processing hierarchy which may ordinarily contribute to the final qualia — and this unnatural stimulation might cause the subject to see Martian colours. (3) A crowded grapheme that is not consciously perceived can nevertheless evoke the corresponding colour. 1991) — it is completely uninformative in determining whether synaesthesia is perceptual or conceptual. 1997) and Marks (e. Even though it has been known for over 100 years. Four lines of evidence support this: (1) Synaesthetically induced colours can lead to perceptual grouping. (2) Synaesthetic colours are not seen with eccentric viewing even if the numbers are scaled in size to make them clearly visible.17 Summary and Conclusions Synaesthesia has always been regarded as somewhat spooky.
[17] This point is consistent with current ideas of distributed processing.g.. (4) A colour-blind synaesthete sees colours in numbers that he cannot otherwise see in real-life visual scenes. 2001) but neither inference is justified. segregation and pop-out.
. The results of Stroop-like interference tasks are sometimes cited as evidence for the view that synaesthesia is sensory (Mills et al.. 1975. see also Churchland et al. 2000) have repeatedly emphasized its potential importance for understanding normal sensory function. The implication of this is that the experience of qualia may depend on the activation of the whole visual hierarchy (or a large part of it). This is even more important for synaesthesia than for ordinary psychophysics since the subject is often trying to express the ineffable. Indeed. 1994). interest in this phenomenon has been revived by the intriguing experimental work and theoretical speculations of Baron-Cohen. it has often been thought of as a curiosity — just a quirk based on early childhood memory associations or a mere metaphorical association between different sensory terms. 1999) and sometimes for the conflicting view that synaesthesia is conceptual (Dixon et al. Mattingley et al. Ramachandran & Hirstein. 1998). Second. 1982.. Rather. The six synaptic levels do not form
a static hierarchy wherein neural transformations of one level are passed on to the next level in a conveyor-like fashion.. it has been largely ignored by mainstream neuroscience and psychology despite the fact that both Cytowic (1989. Gray and colleagues (see above).SYNAESTHESIA — PERCEPTION. three or four levels are co-active at any one time as expected in a distributed system (we are indebted to an anonymous reviewer for bringing this point to our attention. our psychophysical experiments were the first to prove conclusively that synaesthesia is a genuine sensory phenomenon. not just the pontifical cells at the end of the chain. THOUGHT AND LANGUAGE
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phantom limb pain (also caused by abnormal cross-wiring. The main strength of our psychophysical approach to synaesthesia is that we make systematic predictions instead of relying solely on the subjects’ introspective reports.. Harrison. 2000. More recently. Stroop interference merely shows that the association between the grapheme and the colour is automatic. Since Stroop-like interference can occur at any stage in the system — from perception all the way up to motor output (MacLeod.

We suggest that in them the cross-activation occurs at a higher level — perhaps between the angular gyrus (known to be involved in abstract number representation) and a higher colour area in the vicinity that receives input from V4. if the higher colour area has different psychophysical properties then the induced colours will also have different psychophysical properties in higher synaesthetes. One prediction would be that higher synaesthetes should experience colours even with tactile numbers or subitizable clusters of dots. In addition to the lower synaesthetes (JC and ER) there also appear to be other types of number–colour synaesthetes in whom the effect may be more concept driven. We postulate that at least four
. If expressed in the angular gyrus someone may be a higher synaesthete. the colours may not fall off with eccentric viewing of numbers and the induced colours may not give rise to grouping or pop-out.e. but we must bear in mind that if the gene is expressed in a patchy manner in multiple locations. the effect is conceptual rather than sensory. then cross-activation of brain maps may be the basis for metaphor and this would explain the higher incidence of synaesthesia in artists. in higher synaesthetes. that the study of synaesthesia can help us understand the neural basis of metaphor and creativity. poets and novelists (whose brains may be more cross-wired. We suggest. we propose a specific testable hypothesis: That grapheme–colour synaesthesia is caused by a mutation causing defective pruning and cross-activation between V4 (or V8) and the number area. For example. Although the cross-talk idea has been around for some time. if expressed between primary gustatory cortex and adjoining hand and face regions of primary somatosensory cortex. lower synaesthetes. nor would numbers rendered invisible by crowding evoke colours in these higher synaesthetes. These predictions are all easy to test. can lead to more extensive cross-wiring in their brains. however. or both. which lie right next to each other in the fusiform gyrus. We suggest. Furthermore. RAMACHANDRAN & E.S. the result might be a person who ‘tastes shapes’. Our speculations on the neural basis of metaphor also lead us to propose a novel synaesthetic theory of the origin of language. If concepts are represented in brain maps just as percepts are.M. It remains to be seen whether days of the week and months of the year — embodying the abstract rule of cardinality or sequence — would evoke colours only in higher synaesthetes.. also. Perhaps the same mutation that causes crosswiring in the fusiform. i.28
V. no specific brain areas have been suggested and the idea is usually couched in vague terms that do not take advantage of known patterns of localization. if expressed very diffusely. further. so there may be mixed types who combine features of several different types of synaesthesia. that synaesthesia is caused by a mutation that causes defective pruning between areas that are ordinarily connected only sparsely. The distribution may not be bimodal. HUBBARD
Having established the sensory nature of synaesthesia in our first two subjects. If it is expressed only in the fusiform someone may be a lower synaesthete. there might be mixed synaesthetes who complicate the picture. giving them greater opportunity for metaphors). Various transcription factors may then influence the exact locus and extent to which the gene is expressed. And.

that it evolved exclusively as a specific adaptation for communication. but this seems to have escaped the notice of even sophisticated psycholinguists. rather than the exception.. The mutation-based hyperconnectivity hypothesis may also explain why many synaesthetes exhibit such strong emotional reactions to even trivial sensory discord or harmony. 1994): First. given that their syntax is intact? Another intriguing question is whether the hierarchical structure of tool use in early hominids provided an exaptation for the hierarchical structure of syntax (Greenfield.SYNAESTHESIA — PERCEPTION. by the recently discovered mirror neurons system in the ventral premotor area that must represent the movements of others. Instead. 1991).. in evolution. Such hyperconnectivity (whether caused by genes or by TLE-induced kindling) would also increase the value of a reward or aversion.. bouba and kiki). perhaps. thereby strengthening pre-existing associative links (this would allow learning to play a role in synaesthesia). Did the latter accelerate the evolution of the former. including vocal movements). ‘teeny’ and ‘little’ for diminutive objects mimed synaesthetically by a small /I/ formed by the lips and a small vocal tract). This seems very plausible to us: e. as we suggest in this essay? Our neurological approach to this problem will be to give non-linguistic logic puzzles to patients with Broca’s aphasia. Once this was in place other selection pressures could kick in to refine it (through the combined effects of symbol manipulation/semantics and of the exaptation provided by the syllabic structure for syntactic deep structure).
. that language simply involves the specific implementation of a more general-purpose mechanism (such as thinking and symbol manipulation) or second. We suggest that this occurs because of hyperactivation of the amygdala.g. who have lost syntax. and cross-activation between motor maps concerned with gesticulation and vocalizations. hammering a nail or stone core could give rise to distinctions such as ‘active’ and ‘passive’ or ‘subject’ and ‘object’. we postulate that language evolved through co-opting and finding novel uses for multiple mechanisms evolved originally for very different functions and by a fortuitous synergistic bootstrapping between these functions. did they co-evolve through mutual bootstrapping. a synaesthetic mapping between sound contour and motor lip and tongue movements (mediated. nucleus accumbens and other limbic structures by sensory inputs. For example. we know that they cannot use ‘if’.g. one wonders whether tool use may have even provided an exaptation for thought itself. THOUGHT AND LANGUAGE
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earlier brain mechanisms were already in place before language evolved. a non-arbitrary synaesthetic link between object shapes and sound contours (e.g. ‘petite’. Indeed. neither of these extreme views is correct. offline symbol manipulation (including logic). a synaesthetic correspondence between visual appearance and vocalizations (e.
[18] This raises the fascinating question of the relationship between language and thought — more specifi-
cally between the hierarchical/syntactic Chomskyan tree structure and abstract. ‘then’. This would have allowed an autocatalytic bootstrapping culminating in the emergence of a vocal proto-language. ‘but’ and ‘unless’. or was it the other way around? Or. This sort of co-opting of pre-existing machinery for novel uses is the rule.18 This idea is different from the two more traditional theories of language origins (Pinker. On our scheme. but can they play chess (which requires the tacit use of such relational concepts)? Can they still use a computer language or algebra (assuming that they could before the stroke)? And what about patients with Wernicke’s aphasia — can they engage in symbol manipulation and logic. A similar hyperconnectivity (based on kindling rather than mutation) could explain the purported higher incidence of synaesthesia as well as heightened emotions in response to sensory stimuli seen in TLE.

Shakespeare. Patricia Churchland. ‘Anatomy is destiny’ was one of Freud’s few insightful remarks and finds resonance with the main ideas expressed in this paper (e. pp. Belger. the gene is expressed at multiple sites along the sensory processing hierarchy (in a patchy or diffuse manner) including the sensory-to-amygdala connections in some individuals — the limbic system is not the only player. not merely the final stages. Roberts. these results suggest that the entire perceptual pathway (or large portion) is essential for the experience of qualia. In addition. Ramachandran. Jeffrey Gray.S. Puce. Society for Neuroscience Abstracts. synaesthetes can experience qualia that are unavailable to non-synaesthetes. HUBBARD
Our scheme invokes limbic structures for explaining the emotional overtones of synaesthesia but it is very different from Cytowic’s (1989. A. ‘Mu-wave blocking by observation of movement and its possible use as a tool to study theory of other minds’. and colours’. G.A.L. V.. 1997) view that it all happens in the limbic system because the limbic system is phylogenetically ancient and everything must eventually converge on it. Francis Crick. nor even the most important one. Hubbard. all in a single experimental subject. Julia Fuller-Kindy. While both mental imagery and synaesthesia are paradigmatic examples of internal mental states. Vankov.M. we have shown how the cross-wiring hypothesis can explain synaesthetes’ introspective reports. Altschuler.S.
References
Allison. the rare form of pain–colour synaesthesia may be due to cross-wiring in the insular cortex.M.
Acknowledgements
We thank Geoffrey Boynton. E. T. The ideas we have presented in this essay are highly speculative but we hope they will provide a springboard for future speculations and experimental work on synaesthesia. (2000). and 1 F31 MH63585 to E.S. masking.. A. Richard Gregory. (1994).M. we have shown how the relation between the two might be fruitfully explored. E. and perhaps the same might be true for a woman that we recently encountered who reports orgasm–colour synaesthesia). RAMACHANDRAN & E. words. Whether all of our ideas turn out to be correct or not. Mike Morgan and Diane Rogers-Ramachandran for helpful comments. 180.... Nobre. Because neural activation in the fusiform gyrus bypasses normal stages of processing at the retina. Far from being an oddity. William Hirstein. fusiform or angular) to phenotype — systematic psychophysics (e. ‘Human extrastriate visual cortex and the perception of faces. one thing is clear. Nick Humphrey..R.. A. Pineda. including visceral sensations and pain) or the angular gyrus (as discussed above). flicker and so on) — and perhaps even to metaphor. A. In our hyperconnectivity model. p. In addition. synaesthesia allows us to proceed (perhaps) from a single gene to a specific brain area (e.H.g. Cerebral Cortex. Finally.g. 26 (1–2). and the evolution of language. But if one had to choose any single neuroanatomical locus for synaesthesia. we discuss the relevance of this scheme for more subjective aspects of consciousness such as mental imagery and qualia. McCarthy.. E. 544–54.
... This research was funded by NIMH grants 1 RO1 MH60474 to V. better candidates would be the insula (where there is pre-existing convergence of information from many sensory modalities. 4 (5). pop-out fall off with eccentricity. J. numbers.g..30
V.